/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution{
public:
  ListNode *mergeTwoLists(ListNode *l1, ListNode *l2){

    ListNode newHead(0);
    ListNode *pi = &newHead;
    while(l1 && l2){
      if(l1->val > l2->val)
        swap(l1, l2);
      pi->next = l1;
      l1 = l1->next;
      pi = pi->next;
    }
    pi->next = l1 ? l1 : l2;
    return newHead.next;
  }
};